Direct-current converter with current limiting

ABSTRACT

A direct-current converter has an electronic switch which can be made conductive by switch-on pulses and has current limiting by suppression of switch-on pulses. In order to make do with a low-loss current sensor which is easy to realize, a direct voltage corresponding to the peak value of the current is obtained with the aid of an additional switch, in phase with the electronic switch, which is preferably a reverse-operated field-effect transistor. The converter is particularly suitable for applications in which low-impedance short circuits at the output of the converter have to be taken into account.

BACKGROUND OF THE INVENTION

FIG. 1 shows a direct current changer with instantaneous value currentlimiting which is also known as pulse by pulse limiting and is knownfrom DE-PS 26 13 896.

In the single-ended forward converter shown in FIG. 1, the input voltageU_(E) is applied to the capacitor 1 and the output voltage U_(A) to thecapacitator 13. A series circuit formed from the primary winding 91 ofthe transformer 9, the drain-source junction of the field-effecttransistor 6 and the measuring resistor 3 is connected in parallel withthe capacitor 1. The Z diode 5 which serves to limit the voltage isconnected in parallel with the series circuit comprising the measuringresistor 3 and the source-drain junction of the field-effect transistor6. The rectifier diode 10 is connected between the secondary winding 92of the transformer 9 and the capacitor 13. The freewheeling diode 1 isarranged in a shunt arm following the rectifier diode 10. The inductor12 is located in a longitudinal branch between the freewheeling diode 11and the capacitor 13.

The control electrode of the field-effect transistor 6 is connected tothe control device 8 which contains a driver, a clock generator and apulse-width modulator. The control arrangement 8 is controlled both bythe comparator 4 and by the voltage controller 14. The comparator 4 isconnected by its negative input via the reference voltage source 2 toone terminal of the measuring resistor 3 and via its positive inputdirectly to the other terminal of the measuring resistor 3. The voltagecontroller 14 serves to control the output voltage and is thereforeconnected by its actual value input to the output of the converter.

With a circuit arrangement of this kind, adequate current limiting canbe achieved in many cases. However, in the case of converters with arelatively high output voltage, for reasons based on the principle of aso-called "running-out of the characteristic" occurs; that is to say,the limiting effect is reduced or eliminated in the case of a shortcircuit. This can ultimately lead to the destruction of components. Thecause for behavior of this kind lies in the remaining minimum pulsewidth of the switch-off and on pulse which is produced by a series ofdead times in the control chain. If an LC element with an inductor inthe longitudinal arm and a capacitor in the shunt arm is arranged in theoutput circuit and if--due to the aforesaid minimum pulse width--thepositive voltage-time area applied to the output inductor duringswitching on is greater than the negative one in the switch-off phase,the magnetic energy rises and thus the current rises in an unlimitedfashion from period to period. This is also confirmed by the followinginequality which applies in the case of a terminal short circuit:

    U.sub.E ·E·t.sub.Emin >U.sub.DF ·(T-t.sub.Emin)(1)

where

U_(E) =Input voltage

u=Transformation ratio of the transformer

t_(E) min =Minimum switch-on period

U_(DF) =Conductive-state voltage of the freewheeling diode

T=Duration of period. In FIG. 2, the typical operating states areillustrated:

a=Onset of current limiting

b=Threshold case in which the minimum switch-on period still justpermits the limiting effect.

c=Reduced limiting effect in the short circuit due to the condition (1)

As FIG. 2 shows, the problem described cannot be solved with pureinstantaneous value limiting.

It is therefore expedient to provide an additional circuit for currentlimiting in the case of a short circuit.

The invention relates to a direct-current converter with currentlimiting.

A current converter of this kind is already known from DE-B1-28 38 009.

In the case of the known converter, a power switching transistor isdriven by a clock generator with switch-on pulses at a constantoperation frequency. In order to control the output voltage, the pulseduty cycle, that is to say the quotient of the switch-on time of thepower switching transistor divided by the duration of the period, ischanged. In the case of overloading at the output of the direct-currentconverter, the current in the power circuit is limited in order toprotect the components. This is achieved by shortening the switch-ontime. Here, the switching transistor is inhibited with the aid of acurrent measuring circuit as soon as the instantaneous value of thecurrent flowing through the switching transistor, which current ismeasured with the aid of a current transformer, exceeds a predeterminedlimiting value.

Since the storage time of the switching transistor requires a minimumswitch-on time and this minimum switch-on time would require aneconomically unacceptable over-dimensioning of the power circuitcomponents in the case of a short circuit, in the known converter afurther current measuring circuit, which is connected via a diode to theload of the current transformer, intervenes when the minimum switch-ontime of the switching transistor ought to have been undershot and thevoltage at the load of the current transformer has therefore risen bymore than the threshold voltage of the diode. It interposes--ifappropriate, repeatedly--switching pauses lasting for several periods sothat the current in the power circuit does not rise above apredetermined limit. In this way, a larger degree of current limitingcan be achieved.

The additional circuit ensures that the output inductor remains inmagnetic equilibrium and the limiting effect is also maintained in theshort circuit, but is associated with very high expenditure.

SUMMARY OF THE INVENTION

The object of the invention is to construct a pulsed converter of thetype mentioned at the beginning in such a way that a current measuringresistor with the lowest possible resistance is adequate as a currentsensor for active current limiting and a peak value of the current isformed and stored in a low-loss manner. In particular, a low temperaturedependency is intended to be produced with little expenditure.

According to the invention, the converter for achieving this object isconstructed having an electronic switch which is arranged in the maincircuit and connected by its control input to a control circuit andcapable of being alternately switched on and off by the control circuit,having a current sensor arrangement in series with the electronicswitch, having an evaluation arrangement arranged between the currentsensor and a capacitor with a discharge circuit, and having a controldevice which is connected to the capacitor and, in the case of capacitorvoltages lying above a predetermined limiting value, controls thecontrol circuit to the effect of limiting the pulse current flowing inthe main circuit, wherein the current sensor is formed by a measuringresistor and the evaluation arrangement is formed by a furtherelectronic switch and wherein the further electronic switch which isotherwise inhibited can be placed in the conductive state in each caseduring the switched-on phase of the electronic switch arranged in themain circuit.

The further electronic switch is also inhibited during the entireinhibiting phase of the electronic switch located in the main circuit.It can also be conductive during the entire switch-on phase of theelectronic switch located in the main circuit. Since the peak value tobe stored is produced only at the end of the conductive phase, thefurther electronic switch can, if appropriate, be driven in such a waythat it is made conductive only in a temporary subdivision of theswitch-on phase of the switching transistor located in the main circuit,in such a way that both electronic switches are simultaneouslytransferred from the conductive state into the inhibited state. In thisway, switch-on current surges during switching-on of the capacitor canbe reduced, or voltage surges which occur due to switch-on currentsurges of the main circuit at the measuring resistor can be faded out.The invention furthermore has the advantage that a low temperaturedependency is ensured with particularly little expenditure.

In the non-prepublished European Patent Application with the file number89 111 982.8, a pulsed converter is already known which, in addition toan electronic switch fitted in the main circuit, has a furtherelectronic switch which is arranged between a measuring resistor locatedin the main circuit and an RC-Parallel circuit. This additionalelectronic switch is switched on and off in delayed fashion with respectto the first-mentioned electronic switch. The voltage on the capacitorfollows the measured voltage on the measuring resistor. At the time ofthe delayed switching-off of the additional electronic switch, thecapacitor is already discharged.

The data book "SIPMOS-Kleinsignaltransistoren, Leistungstransistoren,Siemens AG, (SIPMOS small signal transistors, power transistors, SiemensAG), Issue 84/85, Order No. B3-3209, pages 24 and 25 admittedlydiscloses that rectifier circuits can be realized with extremely lowconductive state voltages with the aid of field-effect transistors.However, when realizing such a rectifier circuit, a control circuit isrequired which transfers the field-effect transistor into the conductiveor inhibited state as a function of the polarity of the voltage appliedto the field-effect transistor.

It is advantageous that the converter according to the invention doesnot require such a control circuit.

Further advantageous embodiments of the invention include forming thefurther electronic switch by the source-drain junction of a field-effecttransistor and wherein the source-drain junction of the field-effecttransistor is directed in such a way that the reverse diode of thefield-effect transistor is polarized in the conductive direction withrespect to the voltage occurring on the measuring resistor, and whereinthe measuring resistor is dimensioned in such a way that the voltagedropped on it is smaller than the threshold voltage of the reverse diodeof the field-effect transistor.

The electronic switch located in the main circuit is formed by afield-effect transistor wherein both field-effect transistors are of thesame conductance type and wherein terminals with the same polarity ofthe two field-effect transistors and the current measuring resistor areconnected to one another. Furthermore, the gate electrodes of the twofield-effect transistors may be connected to one another.

In a preferred embodiment, the capacitor and a reference voltage sourceare connected in single pole fashion to a reference potential and areconnected by their free terminals to the inputs of a comparator whoseoutput is connected to an inhibiting input of the control circuit. Inaddition, a source for a synchronous triangular waveform signal may bearranged between the capacitor and the comparator or between thereference voltage source and the comparator.

Alternatively, the capacitor and the reference voltage source may beconnected in single pole fashion to a reference potential and may beconnected by their free terminals to the inputs of an operationalamplifier whose output is connected to a pulse-width modulator of thecontrol circuit and inhibits the pulse-width modulator in the case ofcurrent limiting. The output of the operational amplifier may beconnected to a voltage-controlled oscillator of the control circuit soas to reduce the frequency of the oscillator in the case of currentlimiting. The measuring resistor may serve simultaneously as an actualvalue generator of a current controller with superimposed voltagecontrol, in which the actual value of the voltage is the reference valueof the current control.

The invention is explained in greater detail with reference to theexemplary embodiments shown in FIGS. 3, 5 and 6 and with reference tothe pulse diagrams according to FIGS. 4 and 7.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a known converter,

FIG. 2 shows a pulse diagram for the converter according to FIG. 1,

FIG. 3 shows a converter with suppression of switch-on pulses with theaid of a comparator,

FIG. 4 shows a pulse diagram for the converter according to FIG. 3,

FIG. 5 shows a current changer with suppression of switch-on pulses withthe aid of a differential amplifier,

FIG. 6 shows a converter with a voltage-controlled oscillator and

FIG. 7 shows a pulse diagram for the converter according to FIG. 6.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 3, a pulsed converter according to the invention is illustrated.

In the single-ended forward converter shown in FIG. 3, the input voltageU_(E) is applied to the capacitor 1 and the output voltage U_(A) to thecapacitor 13. A series circuit formed from the primary winding 91 of thetransformer 9, the drain-source junction of the field-effect transistor6 and the measuring resistor 3 is connected in parallel with thecapacitor 1. The rectifier diode 10 is located between the secondarywinding 92 of the transformer 9 and the capacitor 13. The freewheelingdiode 11 is arranged in a shunt arm following the rectifier diode 10 .The inductor 12 is located in a longitudinal arm between thefreewheeling diode 11 and the capacitor 13.

An evaluation arrangement, which is composed of the field-effecttransistor 15, the capacitor 16 and the resistor 17 which forms adischarge circuit, is connected to the measuring resistor 3. Thefield-effect transistor 15 is connected by its source electrode directlyto the source electrode of the field-effect transistor 6 and by itscontrol electrode directly to the control electrode of the field-effecttransistor 6. Both field-effect transistors are n-channel MOStransistors.

The capacitor 16 and the reference voltage source 2 are connected to oneanother in single-pole fashion. The comparator 4 is connected by itsinverting input to the reference voltage source 2 and by itsnon-inverting input to the capacitor 16 via the triangular-waveformvoltage source 20. Here, the source electrodes of the two field-effecttransistors 6 and 15 are connected directly to one another and to themeasuring resistor 3.

The control device 8 serves to control the output voltage U_(A). Thecontrol circuit extends from the output of the converter via the voltagecontroller 14, the pulse-width modulator 83, the logic circuit 82 andthe driver 81 to the connecting point of the control electrodes of thetwo field-effect transistors 6 and 15.

The logic circuit 82 and the pulse-width modulator are jointlycontrolled by the clock generator 84. The logic circuit 82 is alsoconnected by an inhibiting input to the output of the comparator 4,which compares the reference voltage U_(S) of the reference voltagegenerator with a composite voltage, which is composed of the voltageU_(C1) applied to the capacitor 16, and the triangular voltage f thetriangular-waveform voltage generator 20. If appropriate, thetriangular-waveform voltage generator 20 can be located in the referencevalue branch instead of in the actual value branch. With suitableselection of the working range, the triangular-waveform voltagegenerator 20 can be replaced by a short circuit.

With the aid of the n-channel MOS transistor 15 driven in synchronismwith the switching transistor 6, the capacitance C1 of the capacitor 16is given low impedance, that is to say, is charged quickly andaccurately to the maximum value of the voltage on the measuring resistor3. The comparatively slow reverse diode 15a, which is unnecessarilycontained in the field-effect transistor 15 and is therefore illustratedby a dashed line, of the MOS-transistor 15 is therefore virtuallywithout effect since its threshold voltage is not achieved in theconducting direction. In the inhibiting phase, the field-effecttransistor 15 very quickly acquires high impedance and prevents reversedischarge. The capacitor 16 can be discharged more or less slowly withthe desired time constant via the resistor 17. Since the residualresistance R_(DSON) of the field-effect transistor 15 is very small withrespect to the resistance value of the discharge resistor 17, thevoltage on the capacitor 16 corresponds very closely to the maximumvalue of the measured current.

The pulse diagram according to FIG. 4 shows this fact. The outputvoltage U_(G) of the driver 81 is composed of switch-on pulses which arefed to the gate-source junctions of the field-effect transistors 6 and15 and switch over respectively at the time t1 into the switch-on stateand at the time t2 into the off state. The voltage U_(i) drops on themeasuring resistor 3 and corresponds to the current i1 flowing in themain circuit of the converter. The voltage U_(C1) is produced on thecapacitor 16. The output voltage U_(KA) of the comparator 4 is asequence of square-wave pulses. The rising edge at time t3 constitutesthe start of an inhibiting signal, the trailing edge at time t4constituting the start of an enable signal.

The brief discharging of the capacitor 16 at the start of the pulse,caused by the sharp rise of the current il is practically insignificantsince the peak value of the current il is not produced until the end ofthe conductive phase and remains stored after switching off. The basicresponse of the measured value U_(i) corresponding to the current il isillustrated in FIG. 4.

The voltage on the capacitor 16 is evaluated according to FIG. 3 withthe aid of a comparator 4. The triangular-waveform voltage source 20supplies an external synchronous triangular-waveform signal which issuperimposed on the voltage U_(C1). A superimposition of this kind of anexternal synchronous triangular-waveform signal which is known per sefrom DE-PS 26 13 896 is advantageous for the evaluation with thecomparator 4. In this way, on the one hand a defined switching criterionis acquired, and on the other hand the stability in the subharmonicrange is increased. In the pulse diagram according to FIG. 4, thistriangular signal is not illustrated for the sake of clarity.

In FIG. 4, the basic signal response for current limiting according touse and in the case of a short circuit is illustrated. The number ofomitted pulses in case c depends on the minimum pulse width and on theconductive state voltage of the freewheeling diode 11.

In the converter according to FIG. 5, an operational amplifier 40 isprovided in place of the comparator 4 in FIG. 3. The output of theoperational amplifier 4 and the output of the current controller 14 areconnected via in each case one diode 18 or 19 to the input of thepulse-width modulator 83 so that a so-called override control isproduced. The driver 81 is connected directly to the pulse-widthmodulator 83.

In this embodiment, the amplified control deviation controls thepulse-width modulator 83 and sets the operating point. In the case of ashort circuit, the amplified control deviation can inhibit thepulse-width modulator 83 for several periods when required and as aresult fully maintain the limiting effect. The converter operatessimilarly to the manner shown in FIG. 4.

A further embodiment is illustrated in FIG. 6. This is particularlyadvantageous in the case of radio-frequency converters. In the case ofoverloading, the operational amplifier 4 amplifies the control deviationU_(S) -U_(C1) and adjusts the voltage-controlled oscillator towardslower frequencies as illustrated in FIG. 7. However, the switch-onduration of the current pulse remains constant. However, due tocontinuous frequency reduction, the desired state of equilibrium is alsoachieved in the case of a short circuit of the output terminals of theconverter.

The converters illustrated in FIGS. 1, 3, 5 and 6 can be set over a verywide frequency range since the field-effect transistor can be a smallMOS transistor with switching times of approximately 5-20 ns and canhave very small parasitic capacitances, e.g. of approximately 5 pF. The,switching frequency can be approximately 1-2 MHz.

The converter can be constructed as a forward converter or as aninhibiting converter with a constant clock frequency or as a converterwith frequency modulation.

Current control with superimposed voltage control (current-mode voltagecontrol), in which the output signal of a voltage controller serves asthe reference value of the current, is readily possible with the aid ofthe measuring signal at the current measuring resistor 3, since thesignal at the low-resistance measuring resistor 3 is virtuallyuncorrupted even in the time periods essential for the control.

It should be understood that various changes and modifications to thepresently preferred embodiments described herein will be apparent tothose skilled in the art. Such changes and modifications may be madewithout departing from the spirit and scope of the present invention andwithout diminishing its attendant advantages. It is, therefore, intendedthat such changes and modifications be covered by the appended claims.

I claim:
 1. A pulsed converter with current limiting comprising: anelectronic switch arranged in a main circuit and connected by itscontrol input to a control circuit and capable of being alternatelyswitched on and off by the control circuit; a current sensor arranged inseries with the electronic switch; an evaluation arrangement arrangedbetween the current sensor and a capacitor with a discharge circuit; afirst control device connected to the capacitor and with capacitorvoltages lying above a predetermined limiting value, controls a seconddevice by limiting a pulse current flowing in the main circuit, whereinthe current sensor is formed by a measuring resistor and the evaluationarrangement is formed by a further electronic switch wherein the furtherelectronic switch which is otherwise inhibited can be placed in theconductive state by the control circuit in each case during theswitched-on phase of the electronic switch arranged in the main circuit,wherein the further electronic switch is formed by the source-drainjunction of a field-effect transistor and wherein the source-drainjunction of the field-effect transistor is directed such that thereverse diode of the field-effect transistor is polarized in theconducting direction with respect to the voltage occurring on themeasuring resistor and wherein the measuring resistor is dimensionedsuch that the voltage dropped at it is smaller than the thresholdvoltage of the reverse diode of the field-effect transistor.
 2. Theconverter as claimed in claim 1, wherein the electronic switch locatedin the main circuit is formed by a field-effect transistor wherein bothfield-effect transistors are of the same conductance type and whereinterminals with the same polarity of the two field-effect transistors andthe current measuring resistor are connected to one another.
 3. Theconverter as claimed in claim 2, wherein the gate electrodes of the twofield-effect transistors are connected to one another.
 4. The converteras claimed in claim 3, wherein the capacitor and a reference voltagesource are connected in single pole fashion to a reference potential andare connected by their free terminals to the inputs of a comparatorwhose output is connected to an inhibiting input of the second controldevice.
 5. The converter as claimed in claim 4, wherein a source for asynchronous triangular-waveform signal is arranged between the capacitorand the comparator or between the reference voltage source and thecomparator
 6. The converter as claimed in claim 3, wherein the capacitorand a reference voltage source are connected in single pole fashion to areference potential and are connected by their free terminals to theinputs of an operational amplifier whose output is connected to apulse-width modulator of the second control device and inhibits thepulse-width modulator in the case of current limiting.
 7. The currentconverter as claimed in claim 3, wherein the capacitor and a referencevoltage source are connected in single pole fashion to a referencepotential and are connected by their free terminals to the inputs of anoperational amplifier whose output is connected to a voltage-controlledoscillator of the second control device and reduces the frequency of theoscillator in the case of current limiting.